Water-soluble cellulose ethers, such as sodium carboxymethylcellulose (NaCMC), methyl-hydroxyethylcellulose (MHEC), methyl-hydroxypropylcellulose (MHPC), ethyl-hydroxyethylcellulose (EHEC), hydroxyethylcellulose (HEC) or hydroxypropylcellulose (HPC) are widely used as viscosity-producing substances (for example, as thickeners, adhesives, binders or dispersing auxiliaries). It is, however, often indispensable to add them to water or aqueous systems, whereby difficulties are liable to occur with respect to dispersing or dissolving these cellulose ethers (without forming lumps) and thus achieving rapid and uniform dissolution until the final desired degree of viscosity is reached. Various methods have been proposed by prior art publications which, in principle, suggest preventing or retarding initial swelling of surfaces of cellulose ether particles (powders, granulates, agglomerates) due to the action of water molecules at least until the particles are uniformly distributed in a water-containing medium.
Cellulose ethers have been treated with dialdehydes, such as glyoxal, during or after a manufacturing process in order to cause reversible cross-linking of the cellulose ether molecules (see, for example, U.S. Pat. No. 2,879,268; German Auslegeschrift No. 12 73 810 (equivalent to U.S. Pat. No. 3,297,583); German Offenlegungsschrift No. 15 18 213 or German Auslegeschrift No. 24 15 556 (equivalent to U.S. Pat. No. 3,997,508). This type of cross-linking has, however, the disadvantage that it is only effective, i.e., makes possible a lump-free addition, in those cases where the cross-linked cellulose ethers are used in aqueous media having pH values close to the neutral point. For this reason, the thus cross-linked cellulose ethers can only be used to a very limited extent in those fields of application where the pH values of the aqueous media are in the highly acidic or highly basic range. In particular in the highly basic range, the cross-linkage, which in general is a semiacetal bond, is rapidly cleaved, so that, as a consequence of this reversible cleavage of the cross-linking, lumps form due to the too rapid interaction between the particle surfaces and the water molecules.
A reversible cross-linking can also be obtained by reacting cellulose ethers with Al.sup.3+ ions [see, for example, German Offenlegungsschrift No. 16 68 854 (equivalent to British Pat. No. 1,224,390) or European Offenlegungsschrift No. 0 038 107], this type of cross-linking being naturally restricted to cellulose ethers with anionic groups, such as NaCMC.
In accordance with German Pat. No. 25 56 754 (equivalent to U.S. Pat. No. 4,097,667), cellulose ethers which carry hydroxyalkyl groups are reacted with chloroformic acid esters, about 0.2 to 0.6 mole of chloroformic acid ester being used per mole of cellulose ether. Hydroxyalkylcelluloses which have been modified in this way can be dissolved by adjusting the pH of the aqueous medium to at least 12 after their addition (as a dispersion). The reaction itself must also be performed in the presence of a base. It is for this reason that the process is less suitable for the treatment of ready-made cellulose ethers, because a relatively high amount of the modifying agent is required. Moreover, the application of a pH of 12 and higher is not without difficulties in the case of certain aqueous systems.
It is said that cellulose ethers modified with boric acid or borates can be added to aqueous media having a pH of more than 10 without agglomeration of the cellulose ethers occurring. According to German Offenlegungsschrift No. 25 35 311 (equivalent to British Pat. No. 1,465,934), it is also possible, if appropriate, to treat cellulose ethers, which are already cross-linked with glyoxal, with borate ions, this modification taking place in a slurry of the cellulose ether in an organic solvent, such as acetone. The maximum amount of added borate ions is about 5 percent. This process is, however, not very well-suited for practical purposes, since relatively high amounts of organic solvent are required, and such solvent has to be removed again from the slurry. Moreover, these modified cellulose ethers cannot be dispersed or at least can be dispersed less readily at a pH of less than 10 than is the case at a pH of more than 10. European Offenlegungsschriften Nos. 0 041 364 and 0 055 820, too, describe the use of borate ions for modifying the solubility characteristics of NaCMC or HEC, but in these cases the borate ions are already employed during alkali cellulose production. Nothing is stated about a possible improved dispersibility at certain pH values.
Cellulose and cellulose derivatives have also been reacted with inorganic or organic silicon compounds: in accordance with German Pat. No. 842 044 (equivalent to U.S. Pat. No. 2,532,622), cellulose esters or cellulose ethers having a DS (degree of substitution) of from 1 to 2.75 are reacted with triaryl- or diarylmonoalkylsilyl halides in the presence of a hydrogen halide acceptor, such as pyridine or ammonia; the products obtained carry the organic silyl groups as ether substituents, and they are said to possess a greater thermal stability and increased hydrophobicity and to be suitable for the production of films; an analysis of the degree of substitution of a cellulose mixed ether based on MC gave a DS.sub.M of 1.7 and a DS.sub.Si of 1.3;
in accordance with U.S. Pat. No. 2,562,955 cellulose, cellulose esters or cellulose ethers are reacted with mono-, di- or trialkylsilyl chlorides or -silyl acetates in the presence of an acid-binding agent, such as pyridine; the products obtained carry the organic silyl groups as ether substituents, and they are said to be insoluble in the solvents and solvent mixtures conventionally used in the field of cellulose ether chemistry; the only cellulose mixed ether based on MC which is described exactly has a DS.sub.M of 1.8 and a DS.sub.Si of 0.24, the products are said to be more hydrophobic and insoluble in organic solvents;
in accordance with German Offenlegungsschrift No. 16 68 129 (equivalent to U.S. Pat. No. 3,418,312) cellulose ethers with trimethylsilyl groups of a DS of between 2 and 3 are prepared by reacting trimethylsilyl chloride, a mono- or disaccharide, an organic solvent, a tertiary amine and cellulose with each other; the products obtained are said to be soluble in organic solvents, such as chloroform or benzene;
in accordance with German Auslegeschrift No. 25 21 946 (equivalent to U.S. Pat. No. 3,961,976) alkali metal cellulose, sodium metasilicate and sodium monochloroacetate are reacted with one another in the presence of H.sub.2 O.sub.2 and Fe.sup.2+ ions, whereby a silicon-modified NaCMC of an undefined chemical structure, which is said to be readily soluble in water, is formed;
in accordance with German Offenlegungsschrift No. 27 25 764 (equivalent to U.S. Pat. No. 4,106,948) an aqueous composition for surface coatings having improved resistance to water is prepared from water, HEC and an organic silicon compound of the general formula R.sup.1 Si(OR.sup.2).sub.3, wherein R.sup.1 is a methyl, ethyl or vinyl group and R.sup.2 is a methyl, ethyl or methoxyethyl group; in particular, this silicon compound is methyl trimethoxysilane; in the mixture, 1 to 30 parts by weight of the organic silicon compound are contained per part by weight of HEC; at a pH between 3 and 5.5 the compositions are clear solutions;
and in accordance with German Offenlegungsschriften No. 31 04 530 (equivalent to British Patent Application No. 2,070,613) and No. 31 04 531 (equivalent to British Patent Application No. 2,070,612) trimethylsilyl celluloses are prepared either in an organic solvent in the presence of Na.sub.2 CO.sub.3 or in liquid ammonia, whereby the degree of substitution obtained in general varies between 1.3 and 2.7; the products are insoluble in water, but may be soluble in certain solvents, such as hydrocarbons or ethers.
However, these known processes still have certain disadvantges, which in most cases are due to the fact that the reacted quantity of silane is very high, and therefore, the cellulose mixed ethers obtained in general are not or are not completely soluble even after prolonged treatment at the most diverse pH values. Moreover, the process conditions of reactions in anhydrous organic solvents, such as pyridine or toluene or liquid ammonia are not suitable for large-scale industrial production of cellulose ethers, since they are much too expensive for the production of bulk products. It is true that reactions of silanes possessing only one group which is able to react with cellulose ethers (such as trimethylsilyl chloride) result, via a monofunctional substitution at the OH groups, in the formation of mixed ethers, but it is not possible to achieve a bi- or trifunctional substitution reaction between molecules (i.e., cross-linking) using this type of silane.